My alimony started in 2018, but changed in 2020. How do I proceed?

It depends on when the actual agreement was made and if the modification was in the original agreement.  If the agreement was before January 1, 2018 you would include it as income on your return.  

You can't deduct alimony or separate maintenance payments made under a divorce or separation agreement (1) executed after 2018, or (2) executed before 2019 but later modified if the modification expressly states the repeal of the deduction for alimony payments applies to the modification. Alimony and separate maintenance payments you receive under such an agreement are not included in your gross income. IRS Topic No. 452 Alimony and Separate Maintenance